Hydrostatically steered vehicles have been available for many years and their application to windrowers has been significant. There has been a limit to the transport speed of these windrowers due to instability at high speed with the drive wheels ahead of the castor wheels (cab forward). Higher speeds on the road allow the user to cover large distances without the use of trailers and tow vehicles.
In co-pending application filed Apr. 28, 2005, Ser. No. 11/116,418 published as 2006/0201732 and Canadian application filed Apr. 27, 2005, Ser. No. 2,513,614 and assigned to the present assignee is shown a machine in which the machine travels in transport mode with the castor wheels leading the drive wheels (engine forward), and in this position the machine is much more stable and higher speeds are possible.
In co-pending application filed Jul. 25, 2005, Ser. No. 11/188,293 published as 2007/0017712 and Canadian application filed Jul. 21, 2005, Ser. No. 2,505,458 and assigned to the present assignee is shown a steering and speed control arrangement for the above machine.
To vary the speed of the machine, both pump displacements are changed simultaneously. To steer the machine, the displacement of each pump is varied relative to each other so that the flow to each wheel is different which causes the machine to turn.
U.S. Pat. No. 6,523,635 (Johnston) issued Feb. 25, 2003 and assigned to Deere discloses a steering and speed control arrangement for a hydraulically driven tractor which includes a quadrant shaped steering rack and a four bar linkage which transmits the pivotal motion of the steering rack to the pump controllers.
U.S. Pat. No. 6,336,051 (Pangels) issued Jan. 1, 2002 and assigned to Carnegie Mellon University U.S. Pat. No. 7,188,015 (Lange) issued Mar. 6, 2007 and assigned to Trimble and U.S. Pat. No. 6,539,303 (McClure) issued Mar. 25, 2003 disclose typical GPS controlled steering systems for providing an automatic output capable of steering a tractor. The disclosures of these patents provide suitable control systems which can be used in the present arrangement and the disclosures of these prior patents are incorporated herein by reference.
In PCT Published Application WO 2006/133546 of Accutrak Systems Limited published Dec. 21, 2006 is disclosed a control system for providing automatic steering of a hydraulically driven tractor where fluid supplied to the drive motors of the driven wheels is by-passed by a control valve to effect the steering action. This arrangement has been found to have a serious problem in that the arrangement loses control in a situation where there is no flow to the motors for example in a slight downhill situation so that the tractor can move uncontrolled to any location.